The present invention relates to the art of nanotechnology, and in particular, to carbon nanotube technology, its function and structure.
A carbon nanotube is a single graphene sheet in the form of a seamless cylinder. The ends of a nanotube typically have hemispherical caps. The typical diameter of a nanotube ranges from about 1 nm to 10 nm. The length of a nanotube potentially can be millions of times greater than its diameter.
Since their discovery in the early 1990s, carbon nanotubes have been the focus of intense study due to their very desirable and unique combination of physical properties. They are chemically inert, thermally stable, highly strong, lightweight, flexible and electrically conductive. In fact, carbon nanotubes may potentially be stiffer and stronger than any other known material.
Carbon nanotubes are currently being proposed for numerous applications, such as, for example, catalyst supports in heterogeneous catalysis, high strength engineering fibers, sensory devices and molecular wires for the next generation of electronics devices.
There has been particularly intense study of the electrical properties of nanotubes, and their potential applications in electronics. Metallic carbon nanotubes have conductivities and current densities that meet or exceed the best metals; and semiconducting carbon nanotubes have mobilities and transconductance that meet or exceed the best semiconductors.
Carbon nanotubes are grown by combining a source of carbon with a catalytic nanostructured material such as iron or cobalt at elevated temperatures. At such temperatures, the catalyst has a high solubility for carbon. The carbon links up to form graphene and wraps around the catalyst to form a cylinder. Subsequent growth occurs from the further addition of carbon.
Current methods of producing carbon nanotubes yield aggregations of nanotubes. Such aggregations are referred to as bundles (or ropes). Bundles typically range in diameter from about 20 nm to 300 nm. Each nanotube in a bundle has its own individual physical properties. For example, the electric property of any given nanotube in a bundle may vary from the extremes of superconducting to insulating. Different end use applications of nanotubes require particular physical properties. Accordingly, it is critical to be able to isolate individual nanotubes and determine their physical properties. Also, the manipulation and tailoring of individual nanotubes are necessary for end use applications. To these ends, methods of exfoliating bundles by dissolving bundles in solvents have been explored.
Raw carbon nanotubes are essentially insoluble in organic and aqueous solvents. Current methods of increasing the solubility of nanotubes are by the derivatization of the nanotubes. For example, acid-shortened carbon nanotubes which have been derivatized with thionyl chloride and octadecylamine were shown to be soluble in several organic solvents (Chen et al. Science 282:95 (1998)). Solubilization has also been achieved by attaching tubes to highly soluble poly(propionylethylenimine-co-ethylenimine) (Riggs et al. J. Am. Chem. Soc. 122:5879 (2000)). Sidewall derivatization with fluorine and alkanes also appears to render tubes soluble in a number of different organic solvents including chloroform and methylene chloride (Boul et al. Chem. Phys. Lett. 310:367 (1999)). Recently, water solubilization has been achieved by derivatization of carbon nanotubes with glucosamine and gum arabic (Bandyopadhyaya et al. Nano Lett. 2:25 (2002)).
The current methods for exfoliating carbon nanotube bundles, and for increasing the solubility of nanotubes, involve time-consuming, complex processes. Also, the range of solvents in which increased solubility has been achieved, and the degree of solubility achieved, are limited. Moreover, current methods of derivatization of nanotubes, in particular sidewall derivatization, destroy the structural integrity of nanotubes, thereby potentially interfering with desirable physical properties. For example, the electrical properties of nanotubes may be eliminated upon such derivatization. These shortcomings of current methods present obstacles for actualizing the utility of carbon nanotubes for end use applications. Moreover, the derivatized nanotubes provided by current methods would require additional synthetic steps in order to use the nanotubes in catalysis or as catalytic supports.
Accordingly, there remains a need for a simple method of exfoliating carbon nanotubes. Also, there is a need for carbon nanotubes which exhibit a high degree of solubility in a wide range of solvents. Moreover, for various end use applications, there remains a need for a method of increasing the solubility of nanotubes without interfering with their intrinsic physical properties.